1. Field of the Invention
The present invention relates to an electronic ballast powering a fluorescent lamp system, and more specifically to an electronic ballast with circuitry for automatically varying the power supplied to a fluorescent lamp system in accordance with varying operating conditions.
2. Description of the Related Art
Electronic ballasts for gas discharge circuits have come into widespread use because of the availability of power MOSFET transistors and insulated gate bipolar transistors ("IGBTs"), which have replaced previously used power bipolar switching devices. Monolithic gate driver circuits, such as the IR2155 sold by International Rectifier Corporation and described in U.S. Pat. No. 5,545,955, the disclosure of which is herein incorporated by reference, have been devised for driving the power MOSFETs or IGBTs in electronic ballasts. The IR2155 gate driver IC offers significant advantages over prior circuits in that it is packaged in a conventional DIP or SOIC package and contains internal level shifting circuitry, undervoltage lockout circuitry, deadtime delay circuitry, and additional logic circuitry and inputs so that the driver can self-oscillate at a frequency determined by external resistors R.sub.T and C.sub.T.
Unfortunately, however, for an electronic ballast with a resonant type output stage (FIG. 1), the frequency of operation of the lamp cannot remain constant. Rather, it is necessary to preheat the lamp at a frequency higher than the resonant frequency, lower the frequency substantially to strike the lamp and, upon lamp ignition, ramp up again to a running frequency. This allows the lamp filaments to be adequately pre-heated before ignition, and allows the voltage across the lamps to gradually increase at a given rate until the lamp ignites and the circuit becomes a low-Q circuit with the lamp running at a given power. Furthermore, if the lamp fails to strike, the gradual increase in lamp voltage and circuit currents allows the half-bridge to be shut off at some predetermined maximum, therefore, avoiding any high currents or voltages which may exceed the maximum ratings of the half-bridge switches, the resonant inductor or resonant capacitor.
It would therefore be desirable to provide a circuit for an electronic ballast which can vary the frequency output by the ballast controller integrated circuit automatically in accordance with the mode of operation (e.g., preheat, ignition, normal operation, shutdown).
In addition to the foregoing, it would be desirable for the electronic ballast circuitry to sense and automatically react to certain fault conditions.
For example, the ballast should first sense if a lamp is present before starting. Additionally, if the lamp is removed or if any of the lamp cathodes should break during running, it is essential that the ballast shutdown (i.e., turn-off the power transistors) to prevent damage to the ballast. If the damaged lamp is then replaced with a functional one, it is desirable that the ballast automatically re-start without the need to manually reset the main voltage at the input.
Prior solutions to sensing if a lamp is present before starting the ballast include a pull-up resistor 204 disposed between the lower lamp cathode and the DC bus voltage (see FIG. 1). If the lamp 202 is removed, then the sensing voltage (i.e., the lamp detection signal) fed back to the driver circuit over line 203 is no longer held `low` by the low-ohmic lamp cathode and is pulled `high` by the pull-up resistor 204. This signal can then be used by a shutdown circuit in the ballast to turn off MOSFETs/IGBTs 206 and 208 and therefore, prevent the ballast from being damaged. If the lamp 202 is re-inserted, the signal is pulled `low` by the cathode resistance and the shutdown circuit frees MOSFETs/IGBTs 206 and 208, and the ballast starts again. This method, however, only senses if the lower cathode breaks. If the upper cathode breaks, the ballast will not shutdown and MOSFETs/IGBTs 206 and 208 eventually will thermally destruct.
In summary, a need exists for an electronic ballast that automatically varies the frequency of the half-bridge circuit depending upon the operating mode, and which furthermore senses a variety of potentially catastrophic conditions, and shuts down upon the occurrence of such conditions.